The organization of the apical membrane of cultured kidney cells (MDCK) has been studied by high-resolution light microscopy. The distribution of lipids and proteins has been determined by labelling with fluorescent dyes. Two proteins have been identified on the apical membrane surface and their distribution has been characterized as a function of cell development. The primary cilium has been shown to function as a flow sensor and to play a crucial role in the genesis of autosomal dominant polycystic kidney disease.